Unless a grounding path is provided, moving vehicles experience a build-up of static electricity that can be of relatively high voltage. Such charge build-up is undesirable for a number of reasons. For example, the presence of such a charge can have an adverse effect on the vehicle's electronic circuitry including radio reception. A charge can create a spark potential that can present a safety hazard during refueling. The grounding of a charge through a vehicle occupant, typically upon entering or exiting the vehicle, can be particularly uncomfortable.
The tendency of a charge to build on a vehicle can be dissipated by providing a continuous ground to earth. Previously, particularly for larger vehicles, a ground was provided by dragging along a chain or other electrical conductor connected to the frame of the vehicle. As the vehicle moved, a charge was conducted to the ground along the chain.
Tires can also be used to provide a ground that dissipates an electrical charge. However, not all materials that might be used in tire construction are necessarily electrically conductive. Rubber compositions that are electrically conductive are generally constructed from compounds having significant proportions of conductive carbon black. Conversely, rubber compositions that are relatively nonconductive tend to have significantly larger amounts of silica relative to carbon black. In general, increasing the relative proportion of silica relative to carbon black decreases conductivity. However, the size and type of the carbon black particulates and the amount of carbon black loading also affects the level of conductivity.
While silica based compositions are generally poor conductors of electricity, the use of silica as a reinforcing material in the tread can provide increased braking ability under wet conditions and also can result in a tire having decreased rolling resistance. Accordingly, methods of providing a path of electrical conductivity through an electrically insulating tread have been suggested. For example, see U.S. Pat. Nos. 6,220,319 or 6,367,525. However, these constructions are not acceptable where a manufacturer desires to use a tread constructed from a material that is electrically conducting. Furthermore, if a manufacturer desires to use nonconducting compositions at other layers of the tire other than the tread, the tire may not provide a ground regardless of a conducting tread.